The present invention relates to radioactivity well logging and is particularly directed to methods and apparatus for calibrating radioactivity well logging tools having a gamma ray detector. The invention is particularly useful for calibrating logging instruments that measure the natural radioactivity of earth formations.
The surveying of earth formations by detection of the natural radioactivity of said formations is presently well known and in widespread use. Such surveying is generally accomplished by moving a radiation detector through a well bore and establishing a record of the natural gamma radioactivity as a function of the borehole depth. This record can then be used to determine the interfaces between different formations, to relate formations observed from one borehole with formations in the same field that have been observed from other boreholes, and to provide depth references within a borehole.
It is also presently well known to detect the natural radioactivity of an earth formation within several appropriately selected energy windows and to combine the measured count rates in each window in order to determine the amounts of uranium, thorium and potassium present in the formation. The technique is described in U.S. Pat. No. 3,976,878, issued Aug. 24, 1976 to P. Chevalier et al. and assigned to the assignee of the present invention.
The natural gamma radioactivity of earth formations can further be used to provide quantitative shale indications. Typically, the count rate that reflects the natural radioactivity of a formation is normalized to a standard natural gamma radiation count rate observed in pure shale. The normalized signal is then considered as reflecting the shale concentration. Such a technique is disclosed in U.S. Pat. No. 3,786,267 to O. Y. Liu et al., issued Jan. 15, 1974 and assigned to the assignee of the present invention.
The use of natural radioactivity measurements to compare radioactive levels between wells, to determine the concentrations of uranium, thorium and potassium and to provide quantitative shale measurements requires that the instruments for performing such measurements be accurately calibrated.
Before being sent to the field, a natural radioactivity well logging instrument may be calibrated in a pit which simulates an earth formation and is made of concrete or cement blocks containing known amounts of uranium, thorium and potassium. The radioactive zone is thick enough to appear infinite to gamma ray detectors. Typically, the thickness of the pit wall is set at two feet.
Field calibration of the natural radioactivity logging tools is generally accomplished by placing a standard gamma ray source (typically of radium 226) at a standard distance (typically 53 inches) from the detector and adjusting the gain of the system so as to obtain a predetermined reading on a measuring apparatus. Unfortunately, the accuracy of this calibration technique is strongly affected not only by the asymmetry of the detector, but also, and chiefly, by a scattering effect due to the tool itself and to its surroundings (drill pipes, catwalks, etc.) that also receive gamma rays which may be redirected toward the detector. As a consequence of these effects, the calibration errors may reach 10%. For this reason, although natural gamma ray logging has been used by the oil industry for more than thirty years, natural radioactivity logs have never been in widespread use for quantitative measurements. A need exists, therefore, for a more accurate field calibration of the natural radioactivity logging tools. Moreover, as the calibration source is positioned at a relatively long distance from the detector, it must be relatively strong (100.mu.Ci) and can thus be a health hazard.
The above-described calibration pit avoids both the undesired scattering effect from nearby objects and the errors due to the asymmetry of the detector, but it is obviously not portable and therefore cannot be used as a field calibrator. It must be added that the purpose of a calibration pit is to simulate an earth formation, whereas the purpose of a field calibrator is to produce a specified count rate when in place, to check that the detection system is performing satisfactorily.
It is therefore an object of the present invention to provide novel methods and apparatus for more accurately calibrating in the field a natural radioactivity logging tool or, more generally, any radioactivity logging tool having a gamma ray detector.